Sensor devices are particularly important in the technical field of drive and control system safety for e.g. machine tools or production machinery, etc. In a machine, a motor is used e.g. to change the position and speed or angular velocity of a tool. In order to detect such a position or speed, e.g. magnetic or optical sensor systems are disposed directly on the motor. For example, optical sensor systems comprise a light emitting diode which transmits light to two phototransistors via a slotted glass disk which rotates with the motor. The signals of the phototransistors are the output signals of the sensor system and are in particular sinusoidal and cosinusoidal. These signals must be evaluated such that at least one actual position value for the tool is produced. In addition, the rpm or speed of the tool must be determinable from the position and the sampling time. The setpoint position value is predefined by the geometry of the workpiece to be machined. The position controller uses the actual position value to adjust the setpoint position value.
To generate the actual position value, the sine and cosine curves are interpolated by means of electronics provided for that purpose. An analog-digital converter converts the analog signals into digital signals, and the zero crossing count of the sine or cosine curves is determined in order to ascertain which sine or cosine curve in the order thereof is currently being output by the sensor system. For fine determination of the angle, the sine is divided by the cosine and from this value the arc tangent is ascertained. If the slotted glass disk has e.g. 2048 slots, 2048 sine or cosine curves are obtained per motor revolution. The arc tangent can once again be resolved into e.g. 2048 individual steps, thereby producing approximately four million possible items of information for each revolution of the motor.
In order to reduce any possible susceptibility to malfunction of the electronics, two-channel evaluation is provided whereby the signals are processed by the sensor independently of one another in two similar channels. The actual position value determined by one channel is compared with the actual position value determined in the other channel. If the actual position values coincide within defined tolerances, the actual position value of one of the two channels is used as the output signal. If the actual position values of the two channels do not coincide, a fault indication is produced. As the position control loop cannot operate reliably in such a case, the machine as a whole is stopped.
EP 1 402 987 A2 discloses a method for monitoring an energy source, e.g. a laser, guided by means of a robot as an example of a handling device, wherein the speed of the energy source is measured and evaluated as an exception situation if the measured speed falls below a predefined threshold value. In such a situation the energy source is switched off with the intention of thereby avoiding risks to people and environment. EP 0 742 499 A2 describes an approach for reliable processing of safety-oriented process signals wherein reliable two- or multi-channel monitoring and signal processing of safety-oriented process signals is made possible by integration of monitoring devices and safety circuits that are conventionally installed externally into already existing components of a machine controller.
DE 103 42 390 A1 describes an approach for reliable processing of a position measurement value, wherein the position measurement value of a measurement station is supplied to a first receiver unit of a first receiver assembly on the one hand and to a second receiver unit of a second receiver assembly on the other, and wherein each receiver assembly has two receiver units, of which one receives position measurement values from one measurement station and the other receives position measurement values from another measurement station. To that extent said approach therefore relates only to the input-side processing of position measurement values.